1. Field of the Invention
The present invention relates to a rotating apparatus and a method for manufacturing the rotating apparatus and, more particularly, to a technology for improving the rotation reliability even after the rotating apparatus has been downsized.
2. Description of the Related Art
The demand for rotating apparatuses represented by hard disk drives (hereinafter referred to as “disk drive device”, or “HDD” also) today is to meet the double requirement for further downsizing and reliability of withstanding long-term usage.
Many of the disk drive devices are equipped with a hub on which recording disks are placed. The hub is supported by a base plate via a bearing unit. The bearing unit includes a shaft and a shaft housing member that houses the shaft, and there is formed a radial dynamic pressure groove on at least one of an outer circumferential surface of the shaft and an inner circumferential surface of the shaft housing member. The bearing unit functions as the bearing in such a manner that the dynamic pressure is produced through a lubricant agent present in a formation space of the radial dynamic pressure grooves. The hub is joined to the outer periphery of the bearing unit. Such a disk drive device as described above is disclosed in Japanese Unexamined Patent Application Publication No. 2003-289646, for instance.
To satisfactorily achieve the write and read of data in the disk drive device, it is required that the recording disk be stably rotated relative to a read/write head at high speed. For that purpose, the hub needs to joined at a substantially right angle to the bearing unit while a sufficiently robust binding strength is maintained against the bearing unit.
A method in which the hub is bonded to the outer periphery of the shaft housing member is conceivable as a way for connecting the hub to the bearing unit. When a desirable junction strength is to be ensured using this method, the following specific method is generally employed. That is, a relatively large gap is formed between the hub and the shaft housing member and then a large amount of hardening agent (bonding adhesive) is present in the gap. However, there are cases where the hardening resin present in the gap contracts unevenly in the axial or circumferential direction when the hardening agent hardens. As the hardening agent unevenly hardens in the circumferential direction, the hub tends to be decentered relative to the bearing unit. Also, as the hardening agent unevenly contracts in the axial direction, the hub is more likely to incline relative the bearing unit.
A method in which the hub is simply press-fit to the outer periphery of the shaft housing member is conceivable as another way for connecting the hub to the bearing unit. To ensure the junction strength by the press-fitting only, the so-called press-fitting margin is generally set in the vicinity of 10 μm. However, as the hub is press-fit to the outer periphery of the shaft housing member, the inner circumferential surface of the shaft housing member may contract and get deformed inward. In particular, when the disk drive device is reduced in size, a cylindrical wall portion of the shaft housing member tends to become thinner on account the downsizing and therefore the deformation of the inner circumferential surface of the shaft housing member becomes large. Forming the radial dynamic pressure groove on the inner circumferential surface of the shaft housing member causes the deformation of the circumferential surface of the shaft housing member, which results in the deformation of the radial dynamic pressure groove. That is, an increase in the deformation of the inner circumferential surface of the shaft housing member results in an increase in the deformation of radial dynamic pressure groove. As the deformation of the radial dynamic pressure groove grows larger, the distribution balance of radial dynamic pressure is disrupted and a flow of the lubricant agent is more likely to occur. This causes the operating life of the bearing unit to be shortened considerably. Even in a case where the radial dynamic pressure groove is formed on the outer circumferential surface of the shaft, a distance up to the radial dynamic groove in opposition to the shaft housing member varies due to the deformation in the inner circumferential surface of the shaft housing member. Hence, the distribution balance of radial dynamic pressure is disrupted similarly and the similar failures to the above occur.